The present invention relates generally to metal strip or sheet employed in making rigid can bodies, ends (i.e., lids) and lid tabs, and particularly to a bright strip product that reflects light in a substantially specular manner and possesses the ability to carry in its surface a minute amount of lubricant into lid forming, deep drawing, and ironing processes.
Can manufacturing generally involves one or more drawing operations and one or more ironing operations, in the case of beverage cans. In the drawing process, which generally occurs prior to the ironing process, a (flat) sheet metal blank or disk is stretched and bent into a shallow cup with a cylindrical punch and die. This process may induce undesirable thickness nonuniformities in the cup sidewall. In order to render the sidewall thickness more uniform, as well as to increase the height of the cup so that it eventually becomes a can, the cup is passed through one or more ironing rings in an ironing process. The ironing process is generally conducted in a special machine known as a "bodymaker", which is usually separate from the drawing apparatus. The clearance between the ironing ring and the punch is generally less than the can sidewall thickness so as to ensure that the sidewall thickness is reduced with resultant elongation of the can.
Deep drawing involves further drawing of a cup using a second punch and die to increase cup depth.
In can manufacturing operations, the initial surface microtopography of aluminum or steel sheet alloys has a profound influence on the can forming process and on the resulting appearance of the can surface. This influence is manifested in the physics of the tooling/workpiece interface; this interface forms as a punch tool first engages the sheet (workpiece) to initiate formation of a can body or lid, until the can body or lid is completed. Important aspects of the interface physics during all phases of can forming and lid manufacture are the friction levels, wear debris generation, adhesive metal transfer to the workpiece surface, and tool surface wear. The frictional characteristics influence particularly the deformation of the flat sheet in the process of being plastically deformed under large strains into a partially enclosed, cylindrical shell, which is the can body.
Further, the functional properties of the can surface, such as frictional levels in deep drawing and ironing, can surface uniformity and associated aesthetics, and the degree to which the can surface approaches the ideal condition of specular reflection of light are greatly influenced by can surface microtopography. Specifically, the lay or general direction of surface roughness, the root mean square roughness, average roughness wavelength, and shape and distribution of asperities have all been identified as parameters that significantly affect the surface aesthetics of the final product.
Metal strips and sheets are rolled in rolling mills having work rolls that physically engage the sheet to reduce its thickness. The surfaces of such work rolls are prepared for rolling by grinding operations which lead to a specific average surface roughness (R.sub.a). The work roll that engages the side or face of the sheet that becomes the inside surface of the can generally has a roughness of about 22 microinches. This work roll roughness is transferred to the sheet surface during rolling of the metal strip. The grinding process generally imparts a directional roughness on the roll surface which, when subsequently used to roll sheet, transfers the lay of this roughness to the sheet surface. The lay of the roughness is generally oriented in the direction of rolling. Currently, can sheet manufacturers are generally of the opinion that this longitudinal sheet roughness provides an acceptable appearance and acceptable frictional characteristics for the can making operation. Their opinions are based on their belief that the directional sheet surface roughness will not be significantly apparent to the naked eye after the can is painted and coated with a light base coating, e.g., lacquer.
A longitudinal roughness on a sheet surface, however, still leads to several undesirable affects. This is in regard to can surface aesthetics and in the can manufacturing process itself. Specifically, a random, longitudinal roughness leads to what is known as "bleed through", which are dark, irregular areas on the exterior surface of a decorated side wall of a finished can. These areas, which indicate a significant and often irregular roughness deviation in the can surface roughness, are particularly evident when white and other substantially light colors are employed to paint the can surface. The paint or coating does not properly cover regions of the can surface which have bleed through. Hence, the term "bleed through" appropriately describes this situation since the paint or coating seems to disappear into the surface as if the surface were bleeding to the inside of the can. The poor surface aesthetics caused by bleed through can result in rejection of the cans by the brewery or soft drink customer.
It is generally believed that bleed through results from two major surface defects which simultaneously appear on a can sidewall to varying degrees. The first is the rather common "looper lines" that are generally parabolic-shaped, parallel lines or "thumb prints", which occur on both sides of the can wall at a 90.degree. angle to the direction in which the sheet was rolled. Looper lines, which are shown in FIG. 2 of applicant's drawings, are generally associated with the aforementioned directional roughness lay on the sheet surface and the deformation path through which the sheet is taken during the can making process.
An additional problem associated with directional sheet surface roughness lay is that it promotes a differential friction effect along the individual surfaces of the can in the process of forming the can since the roughness lay curves relatively to the longitudinal axis of the can. The individual surfaces refer to the sheet/die, i.e., sheet/ironing ring interface, which involves the exterior can surface, and the sheet/punch interface, which involves the interior surface of the can. It is known that if the roughness lay is in the direction of motion of the punch of the can making machine that forms sheet material into a cylindrical shell, less lubricant will be entrapped in the punch/sheet interface since the sheet surface allows lubricant to relatively freely flow in the direction of punch movement, and hence a higher frictional force will be present. If, on the other hand, the sheet surface roughness lay becomes perpendicular to the direction of punch movement, thereby forming looper lines, then more lubricant becomes trapped in the interface so that friction forces are less. Because of this differential entrapment of lubricant along each of the can wall surfaces, a variable surface appearance results and is clearly manifested on the exterior surface of the can, which is most readily apparent to the naked eye.
The second major surface defect which contributes to bleed through consists of an irregular surface roughness that randomly appears on the can exterior. This roughness pattern is generally associated with the tribology of the can making process itself as well as the rolling operation. Upon close examination, this second defect consists of a local increase in surface roughness in the form of a random collection of discrete fissures or microcracks in the sheet surface. Such microcracks are typical of metal surfaces that have been worked in a mixed film lubrication regime. A mixed film regime is discussed below. The microcracks generally degrade the reflectivity of the metal surface and the subsequent brightness of the can surface since microcracks diffuse incident light, thereby making the can less desirable to the customer and therefore less marketable.
In the mixed film lubrication regime, part of the forming load is carried by contacts between a tooling surface and surface asperities of the workpiece. The remaining part is carried by a thin, locally continuous film of pressurized lubricant entrapped around the asperity contacts. The tribology of the asperity contacts is considerably different from that involving thin, pressurized lubricant films. In the case where forming loads are transferred by thin films, the tribology of the interface is decided by the physical properties of the lubricant and the kinematics of the can forming process. The tooling surface has little constraining influence on the deformation of the can surface, since the mating surfaces in question are locally separated by a highly compliant lubricant film. Therefore, metal grains near the surface of the can freely move relative to one another since they are not constrained by a rigid tool surface. This leads to an increase in local surface roughness. An analogous phenomenon is the edge surface of a titled deck of cards being shuffled. The result of such a phenomenon is the bright areas and fissure lines shown in FIG. 6 of applicant's drawings.
The surface roughening problem is unique to metal forming processes and has been consistently misinterpreted by those working in the aluminum industry, for example, as being defects resulting from mechanisms such as: agglomeration of aluminum fines into dark surface streaks, pressed or ironed-in debris, surface oxides, ineffective cleaners, and particle accumulation on tooling surfaces.
It is then concluded that two distinct processes lead to the aforementioned surface roughening phenomenon referred to as bleed through in can manufacturing operations. The first is associated with the directional roll surface roughness, which has its origin in the roll grinding process. This roughness is imparted to the sheet surface during rolling under heavy thickness reductions which are typical in can sheet manufacturing. Due to the formation of the flat sheet into a cylindrical shell, a portion of the roughness becomes oriented perpendicular to the direction of the can forming tool (i.e., the punch). This results in the thumb print discussed earlier. The second is that with higher viscosity lubricants, roughening of the workpiece surface results from the differential deformation of individual surface grains of the workpiece and hence to observed microcracks or fissures.
Process lubricants in the can making industry have bulk viscosities in the range of 43 to 130 centistokes at roughly 40.degree. C. Additive components and base materials used in these lubricants can change the overall viscosity dramatically during the course of can manufacturing. Products of chemical degradation can have viscosities which exceed those of the original mixture (e.g., fatty acid soaps). In general, individual additive components and products of chemical degradation may have a substantial influence on the effective lubricant viscosity in the tooling/workpiece interface. The existence of thin lubricant films implies that the lubricant viscosity in the interface greatly exceeds the bulk lubricant viscosity. This drives some areas of the system into a mixed film regime.
Can manufacturers are steadily increasing cupping and body making speeds in order to improve process efficiency. An increase in such speeds also leads to thin films of lubricant locally entrapped between the can sidewall surface and forming die (i.e., ironing ring). Any increase in film thickness causes fissuring in the sheet surface since the surface is unconstrained by the tool surface, as the can surface is separated from the tool surface by the thin film.
More particularly, entrained lubricant thickness in forming operations imposed upon sheet material increases with increasing speed. This is clearly evident from the following relation for the initial, instantaneous, central film thickness h which separates the punch and the sheet surfaces in an axisymmetric stretch forming operation: ##EQU1## where
R=the punch radius,
.mu.=the lubricant viscosity,
U=the speed at which the punch strikes the flat sheet surface,
.sigma.=the engineering plastic flow stress of the sheet, and
d=the initial sheet thickness.
Although the above equation is appropriate for axisymmetric stretch forming, the initial film thickness in a deep drawing operation is similarly dependent upon the listed process parameters. Increasing the speed of the process therefore increases the thickness of the entrained film. A similar increase in lubricant viscosity also produces a thicker lubricant film.